A filter possessing those characteristics is known from document EP 0 608 049.
In that document, electro-magnetic radiation, and more particularly optical radiation from a light source, passes through a cell containing a gas and has its spectral density modified by the presence of absorption lines in the gas.
The optical radiation modified in this way passes through an optical filter of the Fabry-Perot interferometer type constituted by a moving element disposed facing a fixed element. Together the two elements define a cavity between their respective facing surfaces.
The initial thickness of the cavity is selected so that the filter is tuned on the absorption band of the gas.
Downstream from the filter, a detector receives the radiation transmitted by the tuned filter and determines therefrom the energy which is representative of the gas concentration. This energy is a function of the wavelength of the gas.
In addition, a voltage is applied between the two facing surfaces in the cavity so as to displace the moving element and thus vary the thickness of the cavity, thereby having the effect of displacing the spectral transmission of the filter so that it is no longer tuned on the absorption band of the gas.
In this way, the detector detects radiation energy referred to as "reference" energy since it no longer depends on the wavelength of the gas.
Nevertheless, commonly-used light sources are rarely monochromatic and as a result they present a spectral distribution that covers a range of wavelengths on either side of the wavelength .lambda..sub.0 which corresponds to an absorption line of the gas.
In addition, it happens very often that the gas whose concentration is to be 35 determined is not pure, but is mixed with other gases referred to as "parasitic" or "interfering" gases which possess interference lines for wavelengths lying within the spectral distribution of the source.
FIG. 1a shows the spectral distribution as output from the gas cell for radiation which has encountered an absorption line of a given gas for wavelength .lambda..sub.0 and an absorption line of an interfering gas for wavelength .lambda..sub.1.
FIG. 1b shows the spectral transmission of the filter used in document EP 0 608 049 when the filter is tuned on the wavelength .lambda..sub.0 of the gas. In this figure, the spectral distribution of the radiation is shown in dashed lines.
FIG. 1c shows the energy measured (shaded area) by the detector when the filter is in the position shown in FIG. 1b.
In order to obtain a measured energy value which serves as a reference and which is thus independent of the wavelength .lambda..sub.0 of the gas, it appears from the prior art document that by varying the thickness between the two facing elements of the filter, said filter is moved away from the absorption line of the gas.
However, as shown in FIG. 1b, it can happen that the new position of the filter corresponds to the wavelength of an interfering gas present in the mixture. Under such circumstances, the energy value measured by the detector (shaded area) that is supposed to act as a reference will be as shown in FIG. 1e.
If the energy values respectively measured by the detector in the cases shown in FIGS. 1c and 1e are written S.sub.0 and S.sub.1, and if the signal for determining gas concentration is written U, then: EQU U=S.sub.0 /S.sub.1
More generally, this ratio can be expressed in the following form: EQU S.sub.0 /S.sub.1 =(A+Bx)/(C+Dx)
where x represents the concentration of the gas.
To determine x, it suffices to solve the above equation. It can be seen that the accuracy with which x is determined is directly related to the accuracy with which the value U is obtained, giving: EQU .DELTA.U/U=.DELTA.S.sub.0 /S.sub.0 +.DELTA.S.sub.1 /S.sub.1
Unfortunately, when the reference value S.sub.1 is as shown in FIG. 1e, the term .DELTA.S.sub.1 /S.sub.1 is very large and this induces poor accuracy on the available signal U and thus on the determination of x.